Accurately measuring personal exposure to air pollutants remains the greatest obstacle to determining their impact on human health. The need for more precise exposure assessment is particularly evident for children and adolescents whose exposure can vary widely based on their time-activity patterns including time spent outdoors, at home, school, and in vehicles. In order to meet the need of epidemiologists to characterize personal exposure, we have developed and validated in laboratory settings a novel personal sensor capable of measuring, with high spatiotemporal resolution, exposure to ultrafine particles (UFP). Results of an initial field test found the sensor to be mobile, rugged, and able t provide accurate spatiotemporal measurements of personal UFP exposure. Feedback from children participating in the initial field test indicated that barriers to its use included its voume, weight, and noise. Therefore, we now propose a two-phase project to: 1) evaluate the usability of the sensor after reducing its size, weight, and noise while maintaining its previous measurement capabilities (R21 Phase) and 2) deploy the modified sensor to characterize personal UFP exposure for adolescents enrolled in the Cincinnati Childhood Allergy and Air Pollution Study (CCAAPS) and assess the impact of short-term and peak UFP exposure on respiratory health (R33 Phase). The proposed study represents a continued collaboration between tool-developers and environmental epidemiologists and will allow for the iterative refinement of a personal sensor for UFP exposure. In addition, the application of the sensor in a large-scale epidemiologic study during the R33 phase will address a significant research question regarding the association between short-term and peak UFP exposure and respiratory health. Successful completion of the proposed project will result in a new tool for measuring personal UFP exposure in real world settings and demonstrate the value of the sensor to address novel hypotheses through improved exposure assessment.